The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to compositions and methods for completing subterranean wells, in particular, fluid compositions and methods for completion operations during which the fluid compositions are pumped into a wellbore and make contact with subterranean rock formations.
In the course of completing oil and gas wells and the like, various types of fluids are circulated in the wellbore. These fluids will hereinafter be called “process fluids.” Process fluids include, but are not limited to, drilling fluids, spacer fluids and cement slurries. In addition, these fluids typically contain solid particles. Fluid hydrostatic pressure and pumping pressure create a pressure differential between the wellbore and the surrounding formation rock. As a result, the liquid portion of the fluid has a tendency to enter pores in the subterranean rock, migrate away from the wellbore, and leave the solid particles behind. In other words, a filtration process occurs that is commonly known in the art as “fluid loss.”
Excessive fluid loss may have undesirable consequences. For example, as more and more liquid exits the wellbore and penetrates the subterranean rock, the solids left behind may concentrate and form a plug, preventing further fluid flow in the wellbore and terminating the completion process prematurely. Liquid entering the formation rock may interact with minerals such as clays, causing the rock to lose permeability—a condition known in the art as “formation damage.” The rheological and chemical properties of a completion-fluid system may also be sensitive to the ratio between the liquid and solid ingredients. Disruption of the optimal liquid-solid ratio arising from fluid loss may have a detrimental effect on the completion process and cause failure.
Control of fluid loss is particularly important during primary and remedial well-cementing operations. The goal of primary cementing is to pump a cement slurry in the well and fill the annular space between a casing string and the subterranean rock. The slurry may be pumped down through the casing interior and up the annulus, or vice versa. When the cement slurry hardens, it supports the casing in the well and provides a hydraulic seal between formation strata. Fluid-loss control during primary cementing is necessary to control the rheological properties of the cement slurry, to ensure that chemical reactions in the slurry proceed properly, and to obtain a durable hardened cement that will provide hydraulic isolation throughout the life of the well.
Remedial cementing consists of two main procedures—plug cementing and squeeze cementing. Fluid-loss control is particularly pertinent to squeeze cementing. Squeeze cementing is a process for restoring hydraulic isolation. A cement slurry is pumped downhole to seal casing leaks or voids behind the casing that have allowed hydraulic communication between formation strata. Squeeze cementing involves injecting a cement slurry into strategic locations that are often very small. Fluid-loss control is necessary to avoid premature solids bridging, and to ensure that the cement slurry arrives and hardens at the correct location.
Persons skilled in the art will distinguish the fluid-loss process from “lost circulation.” Unlike fluid loss, wherein the liquid phase of the slurry escapes into the formation and leaves the solids behind, lost circulation is the loss of the entire slurry to cracks, voids or fissures in the formation. The fluid attributes required to treat lost circulation differ significantly from those aimed at controlling fluid loss. This distinction is detailed in: Daccord G, Craster B, Ladva H, Jones T G J and Manescu G: “Cement-Formation Interactions,” in Nelson E B and Guillot D (eds.): Well Cementing-2nd Edition. Houston: Schlumberger (2006): 191-232.
Persons skilled in the art will also appreciate that the fluid-loss-additive requirements in the context of well construction are different from those associated with well stimulation. Fluid-loss additives in well-stimulation fluids are usually designed to be non-damaging—that is, they leave little or no impairment of formation permeability in their wake. Such additives often dissolve or melt after a stimulation treatment, leaving little or no residue behind. Formation damage is less relevant in the context of well-construction fluids, mainly because the solids content of these fluids is far higher than that of well-stimulation fluids. In addition, perforating operations are usually performed after well-construction operations. The resulting perforations penetrate sufficiently far into the formation to bypass damage that may have occurred upon exposure to well-construction fluids. Therefore, there is no need for the fluid-loss additives to dissolve or melt.
Most well-cementing operations employ aqueous cement systems based on Portland cement. These systems may contain solid extenders such as fly ash, blast-furnace slag, silica, clays and zeolite minerals. Recently, manufacturers began offering “composite cements,” wherein materials such as fly ash blast-furnace slag and zeolites are either interground with Portland-cement clinker or blended with finished Portland cement.
Other aqueous compositions that are used less frequently include calcium aluminate cement, Class C fly ash, blends of lime and silica, chemically activated phosphate ceramics, alkali activated blast-furnace slags and geopolymers. The term cement is broadly used herein to include these aqueous well compositions as well as hydraulic and Portland-base systems generally. In addition, drilling fluids are available that, after drilling is completed and casing is lowered into the well, may be chemically activated and converted into a cement.
The cement systems described herein, as well as other process fluids, are subject to fluid-loss difficulties. It is therefore desirable to provide means by which fluid loss may be controlled.